The intercontinental transport of O3, PM, Hg, and POPs contribute to serious public health problems and damage to natural and agricultural ecosystems in many parts of the world. O3 and PM also contribute significantly to climate change on regional and global scales.
There is considerable evidence from experimental human and animal studies and epidemiological studies that exposure to ambient O3 concentrations causes adverse health effects which range from minor sensory irritation to premature death.
The highest concentrations of O3 are typically associated with stagnant conditions, when the contribution from intercontinental transport of air pollution is low and the contribution of local and regional sources are most important. However, intercontinental transport has increased baseline O3 concentrations to the point where they exceed thresholds for protection of vegetation in many locations and exceed thresholds for the protection of human health occasionally in some locations. As public health-based air quality standards continue to be tightened based on new health effects research, the contribution of intercontinental transport to concentrations that exceed such standards will continue to increase.
Relatively few studies have tried to quantify the human health impacts of intercontinental transport of O3 specifically. Those studies have focused on the relationship between annual average concentrations and premature mortality and suggest that intercontinental transport can contribute significantly to health impacts of air pollution within a given receptor region. One study based on the HTAP1 multi-model experiments estimated that intercontinental transport of O3 contributes from 20% to more than 50% of O3-related premature adult mortalities in a given receptor region, subject to large uncertainty.
The sum of the health impacts of transported pollution in downwind foreign regions can be larger than the health impacts of emissions in the source region itself. Although the impact on ambient concentrations in downwind foreign regions may be much less than in the source region itself, the total population exposed in those downwind regions is much greater. The HTAP1 multi-model experiments suggest that emission reductions in North America and Europe will avoid more O3– related mortality outside these source regions than within the regions themselves.
O3 causes damage to crops, forests, and grasslands, which has important implications for productivity, biodiversity, and food security. Recent experimental studies on field crops, adult forest stands and different grassland ecosystems have found significant impacts associated with ecologically realistic free-air O3 fumigations that mimic the observed increases in baseline O3 concentrations.
Global crop yield losses of four staple crops due to exposure to O3 are estimated to be between 3% and 16%, depending on the crop, and are valued at $14 billion – $26 billion per year. Based on the HTAP1 multi-model experiments, intercontinental transport may be responsible for about 5% to 35% of the estimated crop yield losses depending on the location, crop, and response function used. However, there is significant uncertainty in these estimates, part of which is due to the limited representativeness of available exposure-response functions based on threshold indices (e.g., accumulated ozone exposure over a threshold of 40 parts per billion (AOT40) and the sum of all hourly ozone concentrations greater than 0.06 parts per million (SUM06)).
O3 contributes significantly to climate forcing, directly as a greenhouse gas and indirectly by damaging plants and inhibiting their natural uptake of carbon dioxide (CO2). Among O3 precursors, widespread decreases in emissions of CH4, CO, and VOCs will decrease net climate forcing.
Decreasing NOx may increase climate warming over decadal time scales because less NOx leads to less hydroxyl radical, increasing the lifetime of CH4, which is a greenhouse gas itself. Over time, the increase in radiative forcing from the increased lifetime of CH4 is greater than the decrease in radiative forcing from decreased O3 formation. Decreasing emissions of CH4, however, will result in decreases in the direct forcing from CH4 and the direct and indirect forcing of O3, affecting the rate of climate change in the coming decades.
The radiative forcing exerted by O3 is not globally uniform, but extends from the location of precursor emissions over regional and intercontinental scales. This inhomogeneous forcing affects climate change at the global scale and at the regional scale, influencing atmospheric heating and dynamics and ultimately patterns of temperature and precipitation. The largest climatic impacts do not necessarily occur where the radiative forcing occurs and may occur downwind of the source region.
For PM, the experimental and epidemiological evidence for effects on mortality is stronger than it is for O3. Although the highest PM concentrations are typically associated with local and regional emission sources, intercontinental transport events associated with forest fires or dust storms do produce concentrations that exceed short-term public health standards. On a longer-term basis, current levels of intercontinental transport of PM interfere with the ability to meet visibility targets for natural surroundings in western North America. Intercontinental transport of PM components other than wind-blown dust or from fires is not usually sufficient to exceed health-based ambient standards.
Only a few studies have tried to quantify the human health impacts of intercontinental transport of fine particles specifically. Those studies conclude that contributions to PM from emissions within a region are expected to be much more important for human health than emissions from intercontinental transport. However, the impacts of transported PM are still significant. Based on the HTAP multi-model experiments, the intercontinental transport of PM has influences on human mortality that are comparable to O3. While O3 is transported among regions more efficiently, the relationship between PM and mortality is stronger. Consequently, the estimated mortalities attributable to PM within each source region are much higher, and the contributions of the three foreign regions to the mortality in a given home region range from 3 to 5%. Of the total mortalities associated with emissions from North America and Europe, 15% and 12%, respectively, are estimated to be realized outside of these source regions.
PM is a significant contributor to climate forcing; intercontinental transport influences the distributions of PM and, therefore, the extent and magnitude of its forcing. PM is a mixture containing components that mainly cool, including sulphate and organic aerosols, and black carbon that warms. Anthropogenic emissions of black carbon, CH4, CO, and VOCs are estimated to have caused a climate forcing since 1750 roughly as large as that from anthropogenic CO2. Reductions in PM would improve air quality, but for cooling aerosols, including sulphate, nitrate and POM, this would generally increase warming. Reductions in black carbon would typically benefit both air quality and climate.
Hg differs from other major atmospheric pollutants (e.g., O3 and PM) in that its environmental and health impacts are not directly related to its atmospheric burden. While the major redistribution of Hg is via the atmosphere, its primary environmental and health impact is in aquatic systems and for aquatic organisms and their consumers. Atmospheric Hg that is deposited directly or indirectly into aquatic systems is converted from an inorganic form to methylmercury (MeHg) by microbes in the water and sediments of wetlands, lakes, reservoirs, rivers, estuaries and oceans. Unlike other forms of Hg, MeHg biomagnifies in aquatic food webs. Consumption of fish or other aquatic organisms with elevated MeHg concentrations is the primary route of exposure for humans and wildlife.
Thus, to understand the main impact that emission controls will have on MeHg exposure over intercontinental scales, it is necessary to understand the linkages among atmospheric and oceanic transport, methylation in marine ecosystems, exposure and biomagnification in migratory marine fish, the capture and international trade of seafood, and seafood consumption patterns. These linkages are presently poorly quantified.
Persistent Organic Pollutants
By definition, POPs are persistent, bioaccumulative, and toxic. Their adverse effects on human health and wildlife range from various forms of acute toxicity to chronic effects, including carcinogenicity and developmental and reproductive effects. It is the chronic effects from low dose exposures that are most relevant with respect to the impacts of intercontinental transport.
Similar to Hg, POPs are widely distributed through atmospheric transport, but their primary environmental impacts are realized through the contamination of food webs. There is little information about long-term trends of POPs in food or human media outside of western Europe, North America, and Japan, making it difficult to characterize the global impacts of POPs on human health. In addition to elevated human exposures, studies have documented elevated concentrations of POPs in wildlife in remote environments.